Support YAML as an output format.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>

lib/newfol/filemanip.py

@@ -421,6 +421,17 @@ class JSONFile(FileFormat):
                     "(unknown dict)")
         return o
 
+class YAMLFile(JSONFile):
+    """A serializer for YAML.
+
+    Since by default the JSON output by JSONEncoder is valid YAML, this
+    serializer works by outputting JSON.  However, loading YAML is not
+    supported, since that would require a full YAML parser (and has security
+    implications).
+    """
+    def load(self):
+        raise NotImplementedError("Loading arbitrary YAML is not supported.")
+
 class FileStorage:
     """A file (or file-like object) in a certain format."""
     def __init__(self, fmt, filename, txnformat=None, options=None):
@@ -433,6 +444,8 @@ class FileStorage:
             self._backend = PickleFile(filename, self._txn, options)
         elif fmt == "json":
             self._backend = JSONFile(filename, self._txn, options)
+        elif fmt == "yaml":
+            self._backend = YAMLFile(filename, self._txn, options)
         else:
             raise ValueError("{0}: not a supported backend".format(fmt))
     @staticmethod